Discussion Uranus and Neptune mission concepts

[K_Jameson]

I assume you're talking about the 2018 launch window?

Of course! The 2019 window is safe, to me.

How much would the Neptune be able to get with a 7.07 km/s injection burn?

Neptune-1B with Fregat upper stage can theoretically deliver only 300 kg at that velocity. Including some security margin i think that the capability can drop to 150 kg or little more.

 

[Nicholander]

Hmm... So assuming that we're using the a copy of the "Pope" 90 kg probe, that leaves only 60 kg for everything else, assuming we include some security margin. Unless, however, some radiation protection equipment is included and it flies the 2018 launch window instead. Or maybe a little less security margin is used, bringing the payload up to 200 kg or 250 kg. Also, how much isp does the Shakespeare, Efesto Orbiter and Galileo II's engines have? (Assuming that one of the same engines on one of those probes is used for the Neptune probe and flyby mission)

 

[K_Jameson]

Maybe I can do some in-sim test. My calculations are often a little conservative.

On scaling down your probe on Pope dimensions:
Pope is conceived as a very limited spacecraft with only three scientific instruments; its direct, minutes-lasting measurements are intended to be complementary to the remote and long-term orbiter investigations made by the Shakespeare orbiter. An entire mission only on a Pope-class spacecraft isn't worth of the effort, IMHO!
Moving on a more powerful rocket? Or adding some solid kick stage as fourth stage? I can made some calculation and find the right model of kick stage that can fulfill your needs.

Efesto, Galileo II and Shakespeare shares the same (fictional but realistic) engine: dual-mode bipropellant N2O4/MMH, 980 N of thrust and 330 sec of ISP. Shakespeare and Efesto has one of them; Galileo II has four.

 

[Nicholander]

I see, the Pope doesn't get that much science. But I was thinking for a low cost mission to Neptune, that's why it's launched on a Neptune 1B, not a Jarvis. However, the kick motor does sound quite a bit promising, so how about making this Neptune atmospheric probe (Okay, saying "Neptune atmospheric probe" is a bit of a mouthful, so I'll just call it NAP for now until we get a proper name) say, 150 kg to have a larger science package? Or maybe 180 kg? Also, how much do you think the cruise stage would weigh? Keep in mind it needs these things:

RTGs for power
Medium/High gain comm. dishes for communication with Earth
Low gain antennae for communications with the NAP
RCS control thrusters for changing attitude
Separation mechanism for the NAP
Camera(s) for both navigation and taking pictures of Neptune to try to make an accurate global map
Neptune science package/equipment
And probably a whole bunch of other stuff which I can't think of off the top of my head.

EDIT: How much would adding payload would adding a kick motor on top of the Fregat (I assume the 2nd stage will do some of the burn, the Fregat will do some perigee passes while burning, and then the kick motor will do the final kick to Jupiter), say perhaps a Star 48B add to the payload to TJI?

 

[K_Jameson]

II see, the Pope doesn't get that much science. But I was thinking for a low cost mission to Neptune, that's why it's launched on a Neptune 1B, not a Jarvis. However, the kick motor does sound quite a bit promising, so how about making this Neptune atmospheric probe (Okay, saying "Neptune atmospheric probe" is a bit of a mouthful, so I'll just call it NAP for now until we get a proper name) say, 150 kg to have a larger science package? Or maybe 180 kg?

Understandable. So, let's try to make up this mission on a Neptune-1B!
A 150 kg probe, for me, can carry something as same science package but being more pressure-resistant, or vice versa can have the same resistence and a pair or three more instruments.
For comparison, the 300 kg Efesto (i'm talking of the overall weight of probe+aeroshell) carries eight instruments: camera, IR radiometer, nephelometer, lightning detector, net flux radiometer, temperature and pressure probe, anemometer, mass spectrometer.

Also, how much do you think the cruise stage would weigh? Keep in mind it needs these things:

RTGs for power
Medium/High gain comm. dishes for communication with Earth
Low gain antennae for communications with the NAP
RCS control thrusters for changing attitude
Separation mechanism for the NAP
Camera(s) for both navigation and taking pictures of Neptune to try to make an accurate global map
Neptune science package/equipment
And probably a whole bunch of other stuff which I can't think of off the top of my head.

It's difficult to answer; ideally, for a light cruse stage, at least the science package must be scrapped almost entirely, saving at least some dozens of kilograms. And the cameras are often the heaviest payloads...
I think that even a minimal cruise stage can weigh about the same of the atmospheric probe. For reference, Pioneer Venus 2 weighs 270 kg, and the Large Probe circa 315.

EDIT: How much would adding payload would adding a kick motor on top of the Fregat (I assume the 2nd stage will do some of the burn, the Fregat will do some perigee passes while burning, and then the kick motor will do the final kick to Jupiter), say perhaps a Star 48B add to the payload to TJI?

A Neptune-1B with a Star 48B fourth stage can put about 800-900 kg on the required 7,07 km/sec injection burn (depending on the security margin that you want assume - the 900 kg option has virtually no margin).

 

[Nicholander]

Wait, 800 kg?!? That's fantastic! So, if we upgrade the NAP to being 200 kg, and the cruise stage is 320 kg (50 more kg because this is has RTGs and a few other things needed for flying in the outer solar system), with an isp of 330, that leaves us with 280 kg of fuel and 1.394 km/s of delta-v. That is WAY more than we need (I say we need about 400 or 300 m/s of dv minimum), so say if we use a NAP that is 300 kg, (As heavy as the Efesto lander), we get 824 m/s of delta-v. Are there any other 4th stage kick motor that can get us even more extra payload?

 

[K_Jameson]

In the 800 kg budget you must consider the weight of the payload adapter - say 50 or 100 kg. Also, in my calculation I haven't considered the weight of the necessary interstage from the Fregat to the Star 48B.
Consequently, 700 kg can be the net weight of the spacecraft.
I want to say that the Neptune-Fregat-Star configuration has never been tested!

 

[Nicholander]

In the 800 kg budget you must consider the weight of the payload adapter - say 50 or 100 kg. Also, in my calculation I haven't considered the weight of the necessary interstage from the Fregat to the Star 48B.
Consequently, 700 kg can be the net weight of the spacecraft.

Even if it jumps down to 700 kg max payload, the delta-v is 392 m/s, still quite a bit. (This is assuming a 300 kg NAP and a 320 kg cruise stage) And if you change the dry mass to 600 kg, perhaps by making the NAP a little lighter, you get 498 m/s of dv. So still, even with the adapters, we can get a large enough payload to the proper trajectory to Neptune.

I want to say that the Neptune-Fregat-Star configuration has never been tested!

I can say however, that all the equipment involved has been tested!

EDIT: Also, could you answer my previous question, are there any other upper stages like the Star 48B that could get a higher payload to our 7.07 km/s trajectory?

 

[K_Jameson]

Well, the immediately ready alternative is the use of a two-stage Fregat, a stack of two of them one above the other. But for a such high energy trajectory, this don't make much difference, and don't allow substantial improvements over the single Fregat. That Star kick stage is the most cheap and viable solution, IMHO. With other space tugs instead Fregat (eg. the Ariane 5 EPS) we have substantially the same performances, I think (i haven't tested it with numbers). Take in account that the Neptune series of rockets was never conceived for such high energy missions towards the gas giants: we are pushing it at its limits.
A more exotic solution can be: throw away Star and Fregat and instead place a cryogenic upper stage; with a Jarvis C third stage we can deliver at least 1,200 kg to the required 7,07 km/sec trajectory, and with margin, but we can barely insert it under the Neptune fairing, and with little space left for the payload. Other stages as the Centaur can provide even better performances, but are incompatible with the Neptune fairing (perhaps the Centaur G/Prime...).
Also, on the reality side of this affair, integrating new stages in a rocket can be a costly exercise, that is in contrast with your "cheap mission" goals.

EDIT
Another solution can be a Briz-M instead the Fregat. But i don't recall if at least a Velcro mesh of this stage is available.

---------- Post added at 11:11 AM ---------- Previous post was at 09:42 AM ----------

Other details on Shakespeare/Pope:

 

[ISProgram]

Another solution can be a Briz-M instead the Fregat. But i don't recall if at least a Velcro mesh of this stage is available.

Well, N_Molson did make one recently...

 

[Nicholander]

I see K_Jameson, the Star 48B really is our best option here. (That keeps it within safety and cost limits) So, how much delta-v will be used for the final course correction maneuver after the NAP's separation? (Let's say it's separated 40 days before arrival) And what should the cruise stage's periapsis over Neptune be?

 

[K_Jameson]

So, how much delta-v will be used for the final course correction maneuver after the NAP's separation? (Let's say it's separated 40 days before arrival)

I don't recall exactly; surely a little Delta-V maneuvre is sufficient for the course correction. You can minimize that maneuvre anticipating the probe release as much as possible. If i recall correctly, Galileo had released the probe five months before the Jupiter arrival. In this case, precision is crucial or you miss the planet.

Take a look on some of the PDF posted above. I'm pretty sure that the Uranus Orbiter and Probe document reports some indications about the data you search. The Uranus data requires little modifications for Neptune, giving the similar mass, but remember that your approach velocity is higher than a probe that must perform an orbital insertion.

And what should the cruise stage's periapsis over Neptune be?

For me, this is not a crucial problem. After the probe release and the data transmission, the fate of the cruise stage is not important. If we want assume that the transmission can't be completed before the flyby, the only thing that must be assured is the cruise stage survival at the flyby, flying past the planet and completing the transmission in the next hours or days.
You can collect additional data installing a neutral mass spectrometer in the cruise stage and flying it in the upper layers of the Neptune's atmosphere until disintegration, exactly as Pioneer Venus 2 did. But you must assure that the probe data are entirely trasmitted on Earth before this.

 

[Nicholander]

Which PDF? But I'm pretty sure that the DV required for such a maneuver would be pretty small, maybe 30 m/s at most from that distance. And I think that a periapsis which get's a close flyby of Triton would be desirable, however. (As long as it doesn't involve crashing into Neptune, obviously) Anyway, how big do you think the NAP and cruise stage will be? I think the NAP will be maybe 1.5 or 2 meters in diameter (Or in other words, a little bit bigger than the Efesto lander) And the cruise stage really depends on how much space the 80 kg of fuel (In this case N204 and MMH propellant) will take up.

 

[K_Jameson]

This:http://sites.nationalacademies.org/cs/groups/ssbsite/documents/webpage/ssb_059323.pdf

A side note about your project. I really appreciate your efforts on developing a "cheap" mission; has a claim of realism that not always the FOI addons have had in this field (we have developed: big rockets, big capsules, big probes, big spaceplanes... all very costly toys in the real world).

Anyway, thinking about a profitable Neptune mission, i'm convinced that such of a mission should be heavily focused on Triton. With the current tecnology, Triton is our only chance to deeply explore a Kuiper Belt Object, with multiple passes or even with a dedicated orbiter. No other KBO offers the same possibility: an approaching probe should consume a prohibitive quantity of fuel to slowing down until orbital capture, given the very weak gravitational field of such an object. All KBO missions are thus doomed on a fast-flyby flight profile (New Horizons is a good example), except a Triton mission.

For an ice giant atmosphere-focused probe, IMHO Uranus could be a preferable choice, giving the relative proximity of the planet (in respect to the outer one) that poses fewer problems and costs in data trasmission and flight management.

 

[Nicholander]

I agree, one of the main focus of interests in the Neptune system is Triton, but this 'cheap' probe's cruise stage has no hope of slowing down to orbital velocity, that would go to a later mission. However, there is nothing wrong with a close flyby of Triton during the flyby. I think that the NAP is worth it for Neptune, but Triton could be a secondary target. (An orbiter could arrive in a later mission)

 

[K_Jameson]

80 kg of N2O4/MMH can be stored in two spherical tanks of about 42 and 39 cm of diameter, for the N2O4 and the MMH respectively. Add a couple of centimeters for margin (tank thickness)

---------- Post added at 02:32 PM ---------- Previous post was at 01:14 PM ----------

However, there is nothing wrong with a close flyby of Triton during the flyby.

If Triton is in the right position, and if your cruise stage carries AT LEAST a near-infrared spectrometer, a camera and a magnetometer, instruments that add several kilograms to your spacecraft.

---------- Post added at 06:26 PM ---------- Previous post was at 02:32 PM ----------

My concepts for a Neptune mission are a little bit more ambitious (and costly, in the FOI tradition :lol:

OPTION 1: a duplicate of Shakespeare/Pope, tentatively named Le Verrier, with a slightly altered scientific payload, with somewhat more focus on Triton.
Also, the atmospheric probe can be removed and replaced with another type of sub-spacecraft for "stereo" investigations of the Neptune environment and interations with Triton.

OPTION 2: a radically new probe, enclosed in an aeroshell for aerocapture in a retrograde orbit (preserving fuel for fast orbital modifications, Triton-targeted), umbrella-like antenna (i hope better fate than Galileo) and a limited but very specialized payload, highly focused on the KBO: VIMS, mapping camera with laser altimeter, gamma ray and neutron spectrometer, magnetometer and possibly a neutral mass spectrometer and a ground-penetrating radar.

 

[Nicholander]

I find your ideas interesting, though remember that my mission is designed to be low cost and launch on the Neptune-1B. (But my mission is in NO way supposed to replace yours at all, think that Le Verrier is launched in the 2020s or 2030s, and my mission is a preceding one launched in 2019) And I don't see anything wrong with adding those few instruments (They're only, say 5 kilograms at most, if that's what you mean by "several kilograms").

And that cruise stage will be smaller than I thought, since the fuel tanks are half a meter at most in diameter. You know what, I think I'll sketch up how I think it will look in Sketch-Up and then post how it looks.

(By the way, I think I got a name for my mission, d'Arrest is the NAP and Adams is the cruise stage)

---------- Post added at 05:43 PM ---------- Previous post was at 03:44 PM ----------

I've finished the Sketch-Up model of the cruise stage (Adams):

The large ring on the top isn't a communication dish, it's where the NAP (d'Arrest), which is 1.5 meters in diameter, will be until it is separated. That cylinder with the 4 panels coming on it is the RTG, and that little dish on the opposite side of the RTG is the communications dish, I'd say it's a medium gain dish, though It's data rate is slow, it can do the job. It will transmit the data from the Neptune flyby just after the flyby. Also, that hole in the side is there to show where the fuel tanks are. It is also just 1.08 meters tall.

(I didn't add the engine because I didn't know how to make it)

 

[K_Jameson]

(They're only, say 5 kilograms at most, if that's what you mean by "several kilograms").

Taking a look on the New Horizons documentation, i have read these numbers: RALPH 10,3 kg; LORRI 8,8 kg. A magnetometer can weigh... uhm... three/five kilograms, but you must add the weight of the magnetometer boom.

Let's say 25/30 kg in total.

 

[Nicholander]

Hmm... But, we did add 50 extra kilograms to the dry mass (We used the Pioneer Venus cruise stage as a base), so how heavy would an RTG be? and a medium gain communication dish? (These extra 50 kg are supposed to be the things needed for outer solar system exploration and the science package)

 

[Urwumpe]

Hmm... But, we did add 50 extra kilograms to the dry mass (We used the Pioneer Venus cruise stage as a base), so how heavy would an RTG be? and a medium gain communication dish? (These extra 50 kg are supposed to be the things needed for outer solar system exploration and the science package)

Depends on how much electricity you need at which time during the mission.

As very coarse rules of thumb:

The longer you cruise until the science mission starts, the more RTGs you need.
The longer the primary science mission lasts, the more RTGs you need.
The more electricity you need on the average, the more RTGs you need.
The more peak electricity you need, the more RTGs you need or enough buffer batteries to compensate.

You can't shut RTGs down - once you have fuelled them, they are running and aging. You also can't reduce their power output, they will also produce electricity and you just have the choice: Use it for a subsystem or turn it into heat.

If you need 450 W electricity in ten years, you need to launch with about 500 W, because in these ten years, the fuel will slowly decay and the thermocouples to convert heat into electricity will also age and become less efficient.